This invention pertains to apparatus and methods for inspecting a specimen, such as a semiconductor wafer or photomask in a dark field inspection system or the like. It also pertains to apparatus and methods for reducing or eliminating from the inspection results the effects contributed by periodic structures on the specimen.
A diverse number and type of inspection systems are available for inspecting samples for defects. One inspection type is referred to as a “darkfield” inspection. Darkfield inspection makes use of light scattered or diffracted by the surface to characterize and examine features of the surface. As used herein, scattered light shall refer to both scattered light and diffracted light.
FIG. 1 is a cross-section view of an illuminated surface used to illustrate aspects of darkfield inspection. An illumination source 101 projects a light beam I (also referred to herein as the incident beam) onto the surface 102 being examined. A portion of the incident beam I is reflected by the surface as the reflected beam R. If the surface 102 areas. In particular, defect detection and analysis are important in semiconductor processing. Defects include, but are not limited to, pits, bumps, scratches, and a number of other features, which mar the surface 102. Thus, the light of an incident beam I is often subject to some degree of scattering. FIG. 1 illustrates a typical incident beam I having a light scattering pattern schematically depicted by a plurality of scattered light rays 103, 104, 105, and 106, which are scattered by a surface defect 108.
Known darkfield inspection tools use detectors to detect the light scattered from the inspection surface. Some designs use as many as three or four distinct and widely separated discrete photodetector elements. Such discrete photodetector element(s) are positioned so that they are not in the path of the reflected beam R. This results in a detection field where the background (the field) is dark. The scattered light received by the detector provides a representation of the surface 102 whereby the surface defects show up as lighter regions against the dark background or field. Hence, the name darkfield scanning.
When a specimen contains periodic structures, such as semiconductor devices on a wafer, these periodic or “array” structures tend to adversely affect the darkfield inspection results. Although these periodic structures are not considered to be defects, these structures result in scattered light during the darkfield inspection. Since the defects also result in scattered light, the scattered light from the actual defect is not easily distinguishable from the scattered light from the periodic non-defect structures. Thus, the periodic structures contribute noise to the scattered light from the specimen, which is analyzed for defects.
One goal is to eliminate or reduce the contribution of noise resulting from periodic structures on a specimen undergoing darkfield inspection. One technique is to place a Fourier filter in the pupil plane to block scattered light produced by the periodic structures. Since the scattering due to the periodic structure results in diffraction peaks in the pupil plane, the Fourier filter, typically implemented as a hard mask, can be designed to physically block these diffraction peaks from reaching the detectors of the inspection system.
In general, there are two kinds of darkfield inspection tools. The first kind illuminates the sample with spot scanning technology and collects the scattering without any imaging optics. The resolution of this kind is determined by the size of the scanning spot. This kind of system is referred herein as a non-imaging darkfield system. The second kind of darkfield inspection tool “floods” the sample with light and collects the scattered energy with a set of imaging optics and imaging detector (such as a CCD or TDI). Unlike the first kind, the resolution of the second kind is determined by the collection numerical aperture (NA). This kind of system is referred herein as an imaging darkfield system.
Although conventional masks for blocking diffraction peaks from periodic structure are effective and leave no adverse side effects for the non-imaging darkfield system, they have several disadvantages for the imaging darkfield system. One problem with a physical blockage type filter for the imaging darkfield system is that the optical response of the physical mask contains significant numbers of side lobes which can extend to 100 um, also referred herein as a “long range ringing response.” There are two major problems with this long range ringing response. First, in today's semiconductor devices, the array or periodic structure regions are usually surrounded by bus regions, which appear bright during dark field inspection. The light in the bus regions leak into the array region, reducing the array region defect sensitivity. Second, the long-ranged ringing response couples the noise in the bus regions with the noise in the array regions. This is highly undesirable because this kind of noise coupling cannot be well characterized, adding uncertainty to the performance of the system.
In light of the foregoing, improved mechanisms for darkfield inspection are needed. It would be especially beneficial to have an imaging darkfield inspection system that substantially reduces or eliminates the long range ringing response produced by periodic or array structures on a specimen.